1. Field of the Invention
The present invention relates to a vehicle brake system in which a total brake force to be applied to wheels in dependence on the braking operation state is attained by the sum of a hydraulic brake force given by a hydraulic brake device and a regenerative brake force given by a regenerative brake device.
2. Discussion of the Related Art
Heretofore, as described in Japanese unexamined, published patent application No. 2006-21745 (equivalent to United States Patent Publication No. US 2005/0269875 A1), there has been known a vehicle brake system, which is provided with a hydraulic brake device 11 for generating a base hydraulic brake force and a controlled hydraulic brake force applied to wheels 23 and a regenerative brake device 12 for generating a regenerative brake force applied to the wheels 23 in correspondence to the braking operation state. In the hydraulic brake device 11, a base hydraulic pressure is generated by a master cylinder 25 in dependence on the braking operation, and the generated base hydraulic pressure is applied to wheel cylinders 30 of the respective wheels 23 which communicate with the master cylinder 25 through oil passages each with a fluid pressure control valve 32 thereon, to generate the base hydraulic brake force on each wheel 23. Further, an electric motor 39 is driven in response to a controlled hydraulic pressure command value to drive pumps 38 which are provided in combination with hydraulic control valves 32 for generating the controlled hydraulic pressures, and the controlled hydraulic pressures are supplied to the wheel cylinders 30 of the respective wheels 23 to apply the controlled hydraulic brake force to the wheels 23.
The fluid pressure control valves 32 are electromagnetic valves each of which can be switched selectively to a pressure difference state that a controlled pressure difference is generated between the master cylinder 25 side and the wheel cylinder 30 side and to a direct communication state that the controlled pressure difference is not generated. As shown in FIG. 8 schematically showing oil passages of a vehicle brake system, each hydraulic control valve 32 has a small-diameter valve hole 32a, which serves as throttle hole when the wheel cylinder pressure is higher than the master cylinder pressure. In FIG. 8, members identical to those in the aforementioned Japanese application are designated by the same reference numerals.
In the vehicle brake system, as shown in FIG. 5 of the aforementioned Japanese application, there occurs a situation wherein at least both of a regenerative brake force and a controlled hydraulic brake force are given. More specifically, a brake ECU 13 detects the fluctuation of the regenerative brake force which is actually generated by the regenerative brake device 12. When detecting the fluctuation, the brake ECU 13 drives a pump 38 of the hydraulic brake device 11 and switches each hydraulic control valve 32 to the pressure difference state to generate a controlled hydraulic pressure by controlling the pressure difference, whereby a controlled hydraulic brake force depending on the controlled hydraulic pressure is generated on each wheel 23 to compensate for a deficiency in the regenerative brake force caused by the detected fluctuation.
In the vehicle hydraulic brake system described in the aforementioned Japanese application, not only the electric motor 39 is driven to drive the pumps 38 but also the hydraulic control valves 32 are switched to the pressure difference state to control the pressure difference in executing the control for applying the controlled hydraulic brake force.
However, when the pumps 38 are driven with a command being issued to make the controlled pressure difference across each hydraulic control valve 32 zero (i.e., with the commanded pressure difference being zero), the pressure on the wheel cylinder 30 side of each hydraulic control valve 32 rises higher than that on the master cylinder 25 side of the hydraulic control valve 32. This causes the valve hole 32a of each hydraulic control valve 32 to work as throttle hole, whereby a pressure difference (drive pressure difference) is generated between the master cylinder 25 side and the wheel cylinder 30 side of the hydraulic control valve 32. Further, this drive pressure difference is also generated when another command has been issued to each hydraulic control valve 32 to make the controlled pressure difference a predetermined pressure.
Accordingly, as shown in FIG. 9, the rotation of the electric motor 39 at a constant rotational speed causes the pumps 38 to be driven, and the drive pressure difference is thus generated, whereby each wheel 23 is given a brake force which is made up by adding a brake force depending on the drive pressure difference to a controlled hydraulic brake force set as target. This results in reducing the brake force which the regenerative brake force should undertake in dependence on the operation state of a brake pedal, by that corresponding to the drive pressure difference, and thus, a problem arises in that the efficiency in utilizing the regenerative energy is lowered.